My bad, I stand corrected on the 8-pin power draw. But I'm fairly certain that the PCI-E specs do dictate how much power can be drawn by the device, according to what connectors are attached to the device. It doesn't encompass the area of chassis/board/supply testing, so the onus is on those designers to make sure their products can supply what they're supposed to.

I believe the 300W number comes from assuming 75W from the slot, plus the additional power provided by the 6 and 8-pin connectors.

The 300W number does come from assuming 75W from the slot, since it's based on backwards compatibility with PCI-E 1.x. However, there is no specification for the maximum amount of power that a card can draw. There is only the ATX spec which dictates how much power can be supplied by each connector, and the PCI-E spec which dictates how much power can be supplied by the slot itself.